Ships have a propulsion device to generate propulsive force for sailing. In general, a single propeller is used in the propulsion device. However, the propulsion device having a single propeller cannot acquire propulsive force from rotational energy of water streams, and thus causes substantial energy loss.
A Counter Rotating Propeller (CRP) type propulsion device is a device that acquires propulsive force from rotational energy without energy loss. In the counter rotating propeller type propulsion device, two propellers installed on the same axis generate propulsive force via counter rotation thereof. A rear propeller of the counter rotating propeller type propulsion device is rotated in reverse with respect to a rotating direction of a front propeller, thereby acquiring propulsive force from rotational energy of fluid caused by the front propeller. Accordingly, the counter rotating propeller type propulsion device may exhibit higher propulsion performance than the aforementioned propulsion device having a single propeller.
The counter rotating propeller type propulsion device includes an inner shaft connected to an engine within a hull, a rear propeller coupled to a rear end of the inner shaft, a hollow outer shaft rotatably installed around an outer surface of the inner shaft, and a front propeller coupled to a rear end of the outer shaft. In addition, the counter rotating propeller type propulsion device includes a counter rotation unit installed within the hull to reverse rotation of the inner shaft and transmit reversed rotation to the outer shaft. A typical planetary gear mechanism is used as the counter rotation unit.
However, in the case of the above-described counter rotating propeller type propulsion device, the hollow outer shaft extending from the counter rotation unit to the rear of the hull by a long length has difficulty in center alignment with respect to the inner shaft upon installation of the counter rotating propeller type propulsion device to a ship. In addition, the long outer shaft needs an increased lubrication area for reduction in friction between the inner shaft and the outer shaft. The counter rotation of the inner shaft and the outer shaft causes shear of a lubrication layer between the inner shaft and the outer shaft, which makes it difficult to realize efficient lubrication.